In an open chest canine preparation, endocardial and epicardial S-T segment elevations were recorded as circumflex coronary pressure was decreased (in steps of 10 mm. Hg) at a constant aortic pressure, heart rate and cardiac output. Recordings were then repeated at a different aortic pressure, heart rate, or cardiac output. As circumflex coronary pressure is decreased endocardial S-T segment elevation gradually increases as coronary pressure is decreased from 70 to 50 mm. Hg and generally fails to increase further. Epicardial S-T segments, however, are depressed as coronary pressure is decreased from 70 to 50 and then rise rapidly as coronary pressure is decreased further.